Gravity waste water disposal systems are well known and have been in use for many years. For example, a U.S. Pat. No. 2,531,721 of Brock discloses a drain attachment that prevents water from backing upwardly through a drain trap and into an area to be drained. The Brock attachment features an externally screw threaded body that is adapted to be threaded into the upper end of a drain trap. The body has a concentric chamber opening outwardly through its lower end and an annular row of parallel drainage openings extending therethrough and communicating with the chamber. A float is suspended from the body and moveable longitudinally with relation thereto on the side adjacent to the chamber. In addition, the ceiling chamber on the float is provided for closing the open end of the chamber when the float is advanced toward the body.
A floating drain seal apparatus is disclosed in a U.S. patent of Fonoimoana, U.S. Pat. No. 4,936,338. As disclosed, a floating drain seal is utilized in combination with a drain formed with a conical bottom surface and a coaxially aligned outlet pipe. A float plug with a complementary conical bottom surface is spaced from the bottom surface of the drain to effect of drainage water therebetween. The float includes a planar top surface cooperative with an annular tubular seal coaxially spaced exteriorly of a circular matrix of openings. The float includes a hemispherical depression formed coaxially into its top surface to receive a hemispherical bottom portion of a seal coaxially aligned with the drain to effect sealing of the drain when a plugged or slowed drain condition backs up drainage water therethrough.
Finally, a sewer relief valve is disclosed in a U.S. patent of Baker, Jr., U.S. Pat. No. 5,209,257. As disclosed, a vent for relieving pressure in a sewage collection system for building characterized by a buoyant float-closure which will easily open in response to fluid pressure to permit escape of gas or liquid from the system upon the build-up in the system of minimum pressure in excess of atmospheric pressure. The float closure is mounted in a threaded plug which is adapted to replace the conventional clean-out plug on the exterior of the building.
Notwithstanding the above it is presently believed that there is a need and a potential commercial market for an improved pressure relief device for releasing pressure in a drain for a sink, shower or tub in accordance with the present invention. There should be a demand for such devices because they are relatively small and relatively easy to install in a drain, easy to remove from a drain and of rugged construction. In addition such devices can be manufactured and sold at a competitive price and are readily manufactured from a readily accessible plastic.